Liquid Crystal Displays (LCDs) have been used for many years to display both static and dynamic images, including video. Unlike displays based upon cathode-ray tube (CRT) technology that has nearly instantaneous pulse-type display characteristics, LCDs provide significantly slower response times. Slow response times can result in visible distortion of dynamic images, particularly when the dynamic portion of the images include rapid movement. Some newer types of LCDs have significantly slower inherent response times, which make this problem more visible and problematic, even at relatively slow rates of image movement. The slow response times create visible distortions such as smearing or dimming, and can make moving symbology essentially unusable.
Significant effort has been made to correct for this slow response time by including response time correction or compensation circuitry in the display control hardware. All known prior attempts have included circuitry to provide a modified LCD command through analyzing previous commands to detect luminance changes, and applying a larger LCD command signal to force the display response to be faster. One solution has involved inserting a finite impulse response filter (FIR) 10 in the command stream, as shown schematically in FIG. 1. FIR filter 10 includes a function fFIR( ) having as inputs current and previous values of x(n), which is the desired luminance or gray scale for a single display element (a red, green, or blue portion of a display pixel). The output of function fFIR( ) is y(n), which is the commanded luminance sent to the display. If function fFIR( ) is linear, the circuit characterized by FIR filter 10 is linear as well and is characterized by the following linear difference equation:
                              y          ⁡                      (            n            )                          =                              ∑                          r              =              0                        M                    ⁢                                    b              r                        ⁢                          x              ⁡                              (                                  n                  -                  r                                )                                                                        (                  Equation          ⁢                                          ⁢          1                )            
In Equation 1, M is the number of previous commands to be used in the circuit, and br is a weighting coefficient for each command to be used. If function fFIR( ) is nonlinear, the circuit characterized by FIR filter 10 is also nonlinear and is characterized by the following equation:y(n)=f(x(n), x(n−1), . . . , x(n−M))  (Equation 2)
If sufficient delay elements are included, as represented by a large value for M, a relatively good history of past desired luminance values is available to determine a command which will create the desired luminance in a short time.
In practice, the delay elements are implemented as banks of memory (RAM), and a memory storage device is required for each display element in a display. Thus, an XGA display will require 1024×768×3 memory locations for each delay element. The width of memory for each memory location is ideally the same as the LCD gray scale control, typically 6 or 8 bits. Thus, a single delay element comprises a memory array of 2.359 Megabytes.
Because of the large number of memory locations required, there has been an effort to determine effective ways to reduce either the number of delay stages or the width of each memory location. Reducing the number of bits stored for each display element reduces output fidelity. Reducing the number of delay elements reduces the ability to select an appropriate command. Most practical implementations include only one or two delay stages, but such a limited number of delay stages or elements in a FIR filter limits the effectiveness of the filter. Methods have also been described to reduce memory width. While such methods have been proven reasonably adequate for low fidelity television images, they have been less successful in providing the high quality dynamic images provided for avionics situational or tactical displays.
FIG. 2 is a graph showing a command value or signal x(n) that calls for an increase in luminance at the beginning of time frame 1. The response time of an uncompensated signal is shown at 22, and the response time of a signal that is compensated using the FIR filter function fFIR( ) is shown at 24. It can be seen that the compensated response approaches the command signal x(n) much faster than the uncompensated signal 22. The decrease in response time is due to the increased filtered command value y(n) between time frame 1 and time frame 2.
A shortcoming of FIR filter 10 is that only current and previous input command values x(n) are used for computing an output command value y(n). FIR filter 10 does not take into account previous output command values y(n). Further, there is no accommodation for how the LCD display element is actually responding to the command values transmitted thereto. Unfortunately, it is very difficult to determine the output of a single display element in a large LCD array.
It is therefore an object of the invention to improve the response time of display elements used in an LCD display or the like.
It is a further object of the invention to reduce the number of delays required in a circuit for improving the response time of a display element.
It is another object of the invention to reduce the amount of memory required in a circuit for improving the response time of a display element.
A feature of the invention is modeling how a display element responds to a display command, and modifying the display command based upon the modeled response.
An advantage of the invention is an improved display element response time using a minimum amount of memory space.